The invention is directed to a surface wave resonator comprising a substrate of piezoelectric material and comprising reflectors applied onto the one substrate side and between which input and output transducers, i.e. what are referred to as interdigital transducers, are situated.
A suitable substrate material is lithium niobate, lithium tantalate and, because of its temperature stability, particularly quartz. The Rayleigh or shear surface wave is employed dependent on crystal section or profile and propagation direction of the wave in the substrate.
The transducers are what are referred to as interdigital transducers and are composed of metallic electrode fingers arranged in alternating fashion and which respectively lie at bus electrodes or bus bars of different polarity. They are usually designed such that they only slightly reflect the surface wave in the range around the resonant frequency of the oscillator even though embodiments having partially reflecting, for example dispersive, transducers can offer advantages under certain circumstances.
The reflectors of the surface wave resonators are usually composed of individual strip-shaped, metallic elements directed perpendicularly to the surface waves -- as seen in the propagation direction of the surface waves -- and are arranged parallel to one another as well as in identical spacing and may also be potentially composed of corresponding grooves worked from the substrate (see, for example, "Surface Wave Filters", Herbert Matthews, John Wiley and Sons, New York, London, Sidney, Toronto, 1977, pages 412-440). Whereas the two outer reflectors R5 and R6 shown in the surface wave resonator 1 schematically shown in FIG. 2 reflect nearly all of the surface wave energy approaching these reflectors (which is important for achieving high quality), a middle reflector, i.e. a reflector arranged between the two transducers IDW5 and IDW6 shown in FIG. 2, is partially reflecting for incident surface wave energy when such an additional middle reflector is provided.
A transfer function as shown, for example, in FIG. 5 in which the amount and phase of a single-pole resonator according, for example, to FIG. 2, are shown, can be achieved by suitable dimensioning of the sub-structures of the transducers and reflectors, taking parasitic elements such as, for example, lead inductances and resistances, into consideration.
In oscillators that, due to their good properties and low costs, use a single-pole surface wave resonator 1 (see FIGS. 1 and 2 -- referred to below in brief as SW resonator) for stabilizing the oscillatory frequency in the oscillator, the signal outfeed has previously occurred in a separate circuit part 4. There are several circuit alternatives that can assume this function of signal outfeed. They have various advantages and disadvantages; what they have in common is that they hardly suppress noise components such as, for example, the second harmonic of the useful signal which are present in the oscillator signal. A subsequent filtering of the outfed signal with a filter element 5 following the outfeed circuit 4 is therefore often necessary.
In the Pierce circuit which is frequently employed for oscillators and that is shown in a block circuit diagram in FIG. 1, the single-pole SW resonator -- which, moreover, is shown in greater detail in FIG. 2 -- is located in the feedback loop of an amplifier 3. The phase-setting element 2 serves the purpose of guaranteeing the phase conditions for the oscillation at the center frequency of the resonator. The overall phase must amount to a whole multiple of 2 .pi.. The illustrated Pierce circuit has proven especially advantageous in the UHF region above 300 megahertz.